Companies including T-Mobile US Inc, Amazon.com Inc and Cox Communications Inc imposed age limits on who could see recruitment ads, limiting some only to people younger than 38, according to the lawsuit, filed in federal court in San Francisco by the Communications Workers of America.

Facebook said in a statement on its website that it does not engage in age discrimination

Several US employers engaged in age discrimination by placing recruitment ads on Facebook targeting younger workers, according to a lawsuit filed on Wednesday by a communications industry labor union.

Companies including T-Mobile US Inc, Amazon.com Inc and Cox Communications Inc imposed age limits on who could see recruitment ads, limiting some only to people younger than 38, according to the lawsuit, filed in federal court in San Francisco by the Communications Workers of America.

"This pattern or practice of discrimination denies job opportunities to individuals who are searching for and interested in jobs, reduces the number of older workers who apply for jobs with the offending employers and employment agencies, and depresses the number of older workers who are hired," the complaint reads.

The lawsuit is the latest example of criticism leveled at Facebook for so-called micro-targeting, a process that has allowed advertisers to choose who sees their ads based on age, interests, race and even such characteristics as whether they dislike people based on race or religion.

Last month, the company said it was temporarily disabling the ability of advertisers to exclude racial groups from the intended audience of ads, and promised to "do better" at policing discriminatory practices.

Facebook, which is not named as a defendant but is accused in the lawsuit of engaging in the practice in its own recruitment efforts, said in a statement on its website that it does not engage in age discrimination.

T-Mobile and Cox said they do not comment on litigation. Amazon said it has corrected some ads.

"We recently audited our recruiting ads on Facebook and discovered some had targeting that was inconsistent with our approach of searching for any candidate over the age of 18," Amazon said.

The complaint included images of employment recruitment ads that when clicked upon by a user, bring up a screen specifying the age group to which the ad was targeted.

"You're seeing this ad because Amazon Fulfillment Jobs wants to reach people ages 18 to 54 who live or were recently near Silver Spring Maryland," one reads.

Another lists T-Mobile's desire to reach people who are interested in customer service, adding, "There may be other reasons you're seeing this ad, including that T-Mobile Careers wants to reach people ages 18 to 38."

Peter Romer-Friedman, a lead attorney on the case, said companies rely heavily on social media for job recruitment, so the targeting harms older job-seekers. Lawyers will seek class action status for the case and plan to add defendants, he said.(Reporting by Sharon Bernstein in Sacramento, California; Additional reporting by Stephen Nellis in San Francisco; Editing by Lisa Shumaker)